TANK VENTING
SIZING PRESSURE – VACUUM RELIEF VALVES
In order to determine the number and size of OCECO Vent Valves, Flame Arrestors, or Vent Units needed for a given tank, it is necessary to ascertain first the amount of breathing the tank will experience. This is comprised principally of:
Pumping breathing resulting from:
- Pumping stock into a tank (i.e., pressure).
- Pumping stock out of a tank (i.e., vacuum).
Thermal breathing resulting from:
- An increase in temperature (i.e., pressure).
- A decrease in temperature (i.e., vacuum).
In addition to filling and emptying rates and temperature variations – the tank capacity and a number of other variables, including the characteristics of the product stored, the specific gravity of the vapor, etc. affect the volume of tank breathing.
| Tank Capacity (42-Gallon Barrels) | Vacuum Venting All Stocks | Pressure Venting-Stocks with Flash Point Above 100° F |
Pressure Venting – Stock with Flash Point Below 100° F |
|---|---|---|---|
| 1,000 | 1,000 | 600 | 1,000 |
| 2,000 | 2,000 | 1,200 | 2,000 |
| 3,000 | 3,000 | 1,800 | 3,000 |
| 4,000 | 4,000 | 2,400 | 4,000 |
| 5,000 | 5,000 | 3,000 | 5,000 |
| 10,000 | 10,000 | 6,000 | 10,000 |
| 15,000 | 15,000 | 9,000 | 15,000 |
| 20,000 | 20,000 | 12,000 | 20,000 |
| 25,000 | 24,000 | 15,000 | 24,000 |
| 30,000 | 28,000 | 17,000 | 28,000 |
| 35,000 | 31,000 | 19,000 | 31,000 |
| 40,000 | 34,000 | 21,000 | 34,000 |
| 45,000 | 37,000 | 23,000 | 37,000 |
| 50,000 | 40,000 | 24,000 | 40,000 |
| 60,000 | 70,000 | 48,000 | 29,000 |
| 70,000 | 48,000 | 29,000 | 48,000 |
| 80,000 | 52,000 | 31,000 | 52,000 |
| 90,000 | 56,000 | 34,000 | 56,000 |
| 100,000 | 60,000 | 36,000 | 60,000 |
| 120,000 | 68,000 | 41,000 | 68,000 |
| 140,000 | 75,000 | 45,000 | 75,000 |
| 160,000 | 82,000 | 50,000 | 82,000 |
| 180,000 | 90,000 | 54,000 | 90,000 |
PETROLEUM AND PETROLEUM PRODUCTS
Pumping Venting Requirements
Recognizing the industry’s need for a simple guide, for general application, the American Petroleum Institute has set up a standard for determining venting requirements, expressed in terms of cubic feet per hour, of air, at standard conditions (i.e., 60° F and 14.7 lbs. per square inch absolute). This standard, which applies to all vapor-tight aboveground tanks storing petroleum and petroleum products that are liquids at atmospheric temperatures and pressures, is as follows:
Tanks containing petroleum products with a flash point above 100° F
Breathing requirement is 6 cu. ft. per hour of air for each barrel per hour of maximum tank filling rate, plus the thermal outbreathing requirement shown in Table 1 (Pressure Venting); also 5.6 cu. ft. per hour of air per barrel of maximum tank emptying rate, plus the thermal inbreathing requirement shown in Table 1 (Vacuum Venting). standards, because the flow capacities of these units are also expressed in terms of air at standard conditions.
Tanks containing petroleum products with a flash point below 100° F
Breathing requirement is 12 cu. ft. per hour of air for each barrel per hour of maximum tank filling rate plus the thermal outbreathing requirement shown in Table 1 (Pressure Venting); also 5.6 cu. ft. per hour of air per barrel of maximum tank emptying rate, plus the thermal inbreathing requirement shown in Table 1 (Vacuum Venting).
The number and size of OCECO Vent Valves, Free Vent Units or Conservation Units needed for a given tank can be selected easily (see following example) when tank breathing capacities are computed from these standards, because the flow capacities of these units are also expressed in terms of air at standard conditions.
EXAMPLE:
To determine the size and number of OCECO V-130 Conservation Vent Valves or Conservation Vent Units needed for the following conditions:
Tank Capacity 20,000 Barrels
Flash Point Below 100° F.
Pumping Rate – In 1,000 bbls. per hour
Pumping Rate – Out 1,200 bbls. per hour
Allowable Working Pressure 2 inches of Water
Allowable Working Vacuum 1 oz. per sq. inch
SOLUTION:
| Pressure (Cu. Ft./ Hr.) | Vacuum ( Cu. Ft./Hr.) | |
|---|---|---|
| Pumping Requirement (From preceding Paragraph titled “Tanks containing petroleum products with a flash point below 100° F” | (12 x 1000) = 12,000 | (5.6 x 1200) = 6,720 |
| Thermal Requirement (From Table 1-20,000 Bbl. Tank Capacity | +20,000 | +20,000 |
| Total Breathing Requirement | 32,000 | 26,720 |
Then turning to the capacity charts or tables with the bulletins describing the OCECO V-130 Conservation Vent Valves and OCECO Conservation Vent Units we find that a 6″ V-130 Vent Valve or and 8″ Conservation Vent Unit will be needed to meet the pressure and vacuum requirements.
VENTING REQUIREMENTS FOR OTHER THAN PETROLEUM PRODUCTS
In order to determine the number and size of OCECO Conservation Valves, Free Vent Units or Conservation Vent Units needed for a given tank, when the tank is storing something other than petroleum or its products, it is necessary to know the pressure and vacuum information listed below:
Pressure or outbreathing information
- Quantity of vapor or free gas to be vented (cu. ft. per hour)
- Maximum temperature of vapor or gas to be vented.
- Specific gravity of gas or vapor at standard conditions.
- Maximum allowable working pressure.
Vacuum or inbreathing information
- Quantity of free air required (cu. ft. per hour).
- Maximum temperature of air.
- Maximum allowable working vacuum.
After the above information has been determined, the values can be converted into equivalent values for air. Use the temperature correction factors in Table II, the specific gravity correction factors in Table III and the formula given below. The selection of the proper number and size of OCECO Valves or Conservation Vent Units can be made directly from the capacity tables or charts which also show values based on standard air.
FOR PRESSURE RELIEF
Formula No. 1
Required std. air capacity = Required vapor of free gas capacity x Ks.g. x Kt x 1.05
Definitions:
- Free gas = Gas at atmospheric pressure
- Free air = Air at atmospheric pressure
- Standard conditions = 60°F. and 14.7 lbs./in.2 absolute
- Standard air = Air at standard conditions
&nbps; = Air at 0.0764 lbs./ft.3 density - Specific gravity of gas = Ratio of unit wt. of gas at standard conditions to unit wt. of air at standard conditions = molecular wt. / 29
- Ks.g = Square root of the Specific Gravity (see Table No. III)
- Kt = Square root of (T+46) / -520 (see Table No. II)
- T = Gas Temperature in degrees Fahrenheit
| Temp. Degrees F. | Kt Correction Factor | Temp. Degrees F. | Kt Correction Factor |
|---|---|---|---|
| 0 | 0.940 | 110 | 1.047 |
| 10 | 0.951 | 120 | 1.056 |
| 20 | 0.961 | 130 | 1.065 |
| 30 | 0.971 | 140 | 1.074 |
| 40 | 0.981 | 150 | 1.083 |
| 50 | 0.990 | 160 | 1.092 |
| 60 | 1.000 | 170 | 1.102 |
| 70 | 1.010 | 180 | 1.110 |
| 80 | 1.019 | 190 | 1.118 |
| 90 | 1.028 | 200 | 1.127 |
| 100 | 1.038 |
| Specific Gravity | Ks.g. Correction Factor | Specific Gravity | Ks.g. Correction Factor | Specific Gravity | Ks.g. Correction Factor |
|---|---|---|---|---|---|
| 0.20 | 0.447 | 0.95 | 0.975 | 1.80 | 1.342 |
| 0.30 | 0.548 | 1.00 | 1.000 | 1.90 | 1.378 |
| 0.40 | 0.632 | 1.05 | 1.025 | 2.00 | 1.414 |
| 0.50 | 0.707 | 1.10 | 1.049 | 2.50 | 1.590 |
| 0.55 | 0.742 | 1.15 | 1.072 | 3.00 | 1.732 |
| 0.60 | 0.773 | 1.20 | 1.095 | 3.50 | 1.871 |
| 0.70 | 0.837 | 1.30 | 1.140 | 4.50 | 2.121 |
| 0.75 | 0.866 | 1.40 | 1.183 | 5.00 | 2.236 |
| 0.80 | 0.894 | 1.50 | 1.225 | 5.50 | 2.345 |
| 0.85 | 0.922 | 1.60 | 1.265 | 6.00 | 2.449 |
| 0.90 | 0.949 | 1.70 | 1.304 |
EXAMPLE:
To determine the size and number of OCECO V-130 Conservation Vent Valves or Conservation Vent Units needed for the following conditions:
- Quantity of vapor to be vented= 24,000 cu. ft. per hr.
- Maximum temperature of vapor to be vented= 160° F.
- Specific gravity of vapor at 60° F.= 1.5
- Allowable working pressure= 2″ of water
- Quantity of air required= 25,700 cu. ft. per hr.
- Maximum air temperature= 120° F
- Allowable working vacuum= 1 oz. per sq. inch
SOLUTION:
Outbreathing, i.e., pressure side
Using formula: Formula No. 2
Required std. air capacity:
= Required vapor or free gas capacity x Ks.g. x Kt x 1.05
or = 24,000 x 1.225 x 1.092 x 1.05
or = 33,800 cu. ft. per hour
i.e., outbreathing requirement is 33,800 cu. ft. per hour.
Inbreathing or vacuum side
Using the formula:
Required std. air capacity = (Required air capacity) x Kt
or = 25,700 x 1.056
or = 27,200 cu. ft. per hour
i.e., inbreathing requirement is 27,200 cu. ft. per hour.
Then, turning to the capacity charts with the bulletins describing the OCECO V-130 Conservation Vent Valves and the OCECO Conservation Vent Units, we find that a 6″ V-130 Vent Valve or an 8″ Conservation Vent Unit will be needed to meet the pressure and vacuum requirements.
Converting Gas Weight to Volume
Whenever calculating venting requirements for products where the gas is given in weight instead of volume. The equivalent volume can be found as follows:
V = 379.5 x (W / M)
where:
V = volume in cu. ft. at std. cond. (60° F and 14.7 PSI),
W = weight product in lbs.,
and M = molecular wt.
Example:
Find the volume at standard conditions of 5 lbs. hydrogen (H2).
Volume = 379.5 * 5 / 2.16 = 878 cu. ft.